flatMap

Stream.flatMap, as it can be guessed by its name, is the combination of a map and a flatoperation. That means that you first apply a function to your elements, and then flatten it. Stream.maponly applies a function to the stream without flattening the stream.

To understand what flattening a stream consists in, consider a structure like [ [1,2,3],[4,5,6],[7,8,9] ] which has "two levels". Flattening this means transforming it in a "one level" structure : [ 1,2,3,4,5,6,7,8,9 ].

아, 설명이 자바 8 ..;;

출처 : http://stackoverflow.com/questions/26684562/whats-the-difference-between-map-and-flatmap-methods-in-java-8